PQ: Can I go swimming when I have my period?

Yes, but a pad won't work underwater, so you need to wear a tampon.
Tampons absorb the menstrual flow from the inside, so being in water won't affect
them.

If you've never worn a tampon before, you may want to talk with your mom or
another woman you trust. You may want to get the slim-fit type, read the directions
first, and leave some extra time to get it properly in place.